Internet Engineering Task Force Nabil Bitar
Internet Draft Verizon
Intended status: Informational
Expires: Oct 2014 Marc Lasserre
Florin Balus
Alcatel-Lucent
Thomas Morin
France Telecom Orange
Lizhong Jin
Bhumip Khasnabish
ZTE
April 15, 2014
NVO3 Data Plane Requirementsdraft-ietf-nvo3-dataplane-requirements-03.txt
Status of this Memo
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Abstract
Several IETF drafts relate to the use of overlay networks to support
large scale virtual data centers. This draft provides a list of data
plane requirements for Network Virtualization over L3 (NVO3) that
have to be addressed in solutions documents.
Table of Contents
1. Introduction.................................................31.1. Conventions used in this document.......................31.2. General terminology.....................................32. Data Path Overview...........................................33. Data Plane Requirements......................................53.1. Virtual Access Points (VAPs)............................53.2. Virtual Network Instance (VNI)..........................53.2.1. L2 VNI................................................53.2.2. L3 VNI................................................63.3. Overlay Module..........................................73.3.1. NVO3 overlay header...................................83.3.1.1. Virtual Network Context Identification..............83.3.1.2. Quality of Service (QoS) identifier.................83.3.2. Tunneling function....................................93.3.2.1. LAG and ECMP........................................93.3.2.2. DiffServ and ECN marking...........................103.3.2.3. Handling of BUM traffic............................113.4. External NVO3 connectivity.............................113.4.1. Gateway (GW) Types...................................123.4.1.1. VPN and Internet GWs...............................123.4.1.2. Inter-DC GW........................................123.4.1.3. Intra-DC gateways..................................123.4.2. Path optimality between NVEs and Gateways............123.4.2.1. Load-balancing.....................................13Lasserre, et al. Expires Oct 15, 2014 [Page 2]

Internet-Draft NVO3 Data Plane Requirements April 20143.4.2.2. Triangular Routing Issues..........................143.5. Path MTU...............................................143.6. Hierarchical NVE dataplane requirements................153.7. Other considerations...................................153.7.1. Data Plane Optimizations.............................153.7.2. NVE location trade-offs..............................154. Security Considerations.....................................165. IANA Considerations.........................................166. References..................................................166.1. Normative References...................................166.2. Informative References.................................167. Acknowledgments.............................................171. Introduction1.1. Conventions used in this document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC-2119 [RFC2119].
In this document, these words will appear with that interpretation
only when in ALL CAPS. Lower case uses of these words are not to be
interpreted as carrying RFC-2119 significance.
1.2. General terminology
The terminology defined in [NVO3-framework] is used throughout this
document. Terminology specific to this memo is defined here and is
introduced as needed in later sections.
BUM: Broadcast, Unknown Unicast, Multicast traffic
TS: Tenant System
2. Data Path Overview
The NVO3 framework [NVO3-framework] defines the generic NVE model
depicted in Figure 1:
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The result of this lookup yields the corresponding information
needed to build the overlay header, as described in section 3.3.
This information includes the destination L3 address of the egress
NVE. Note that this lookup might yield a list of tunnels such as
when ingress replication is used for BUM traffic.
The overlay header MUST include a context identifier which the
egress NVE will use to identify which VNI this frame belongs to.
The egress NVE checks the context identifier and removes the
encapsulation header and then forwards the original frame towards
the appropriate recipient, usually a local VAP.
3. Data Plane Requirements3.1. Virtual Access Points (VAPs)
The NVE forwarding plane MUST support VAP identification through the
following mechanisms:
- Using the local interface on which the frames are received, where
the local interface may be an internal, virtual port in a virtual
switch or a physical port on a ToR switch
- Using the local interface and some fields in the frame header,
e.g. one or multiple VLANs or the source MAC
3.2. Virtual Network Instance (VNI)
VAPs are associated with a specific VNI at service instantiation
time.
A VNI identifies a per-tenant private context, i.e. per-tenant
policies and a FIB table to allow overlapping address space between
tenants.
There are different VNI types differentiated by the virtual network
service they provide to Tenant Systems. Network virtualization can
be provided by L2 and/or L3 VNIs.
3.2.1. L2 VNI
An L2 VNI MUST provide an emulated Ethernet multipoint service as if
Tenant Systems are interconnected by a bridge (but instead by using
a set of NVO3 tunnels). The emulated bridge could be 802.1Q enabled
(allowing use of VLAN tags as a VAP). An L2 VNI provides per tenant
virtual switching instance with MAC addressing isolation and L3
tunneling. Loop avoidance capability MUST be provided.
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Forwarding table entries provide mapping information between tenant
system MAC addresses and VAPs on directly connected VNIs and L3
tunnel destination addresses over the overlay. Such entries could be
populated by a control or management plane, or via data plane.
Unless a control plane is used to disseminate address mappings, data
plane learning MUST be used to populate forwarding tables. As frames
arrive from VAPs or from overlay tunnels, standard MAC learning
procedures are used: The tenant system source MAC address is learned
against the VAP or the NVO3 tunneling encapsulation source address
on which the frame arrived. Data plane learning implies that unknown
unicast traffic will be flooded (i.e. broadcast).
When flooding is required, either to deliver unknown unicast, or
broadcast or multicast traffic, the NVE MUST either support ingress
replication or multicast.
When using underlay multicast, the NVE MUST have one or more
underlay multicast trees that can be used by local VNIs for flooding
to NVEs belonging to the same VN. For each VNI, there is at least
one underlay flooding tree used for Broadcast, Unknown Unicast and
Multicast forwarding. This tree MAY be shared across VNIs. The
flooding tree is equivalent with a multicast (*,G) construct where
all the NVEs for which the corresponding VNI is instantiated are
members.
When tenant multicast is supported, it SHOULD also be possible to
select whether the NVE provides optimized underlay multicast trees
inside the VNI for individual tenant multicast groups or whether the
default VNI flooding tree is used. If the former option is selected
the VNI SHOULD be able to snoop IGMP/MLD messages in order to
efficiently join/prune Tenant System from multicast trees.
3.2.2. L3 VNI
L3 VNIs MUST provide virtualized IP routing and forwarding. L3 VNIs
MUST support per-tenant forwarding instance with IP addressing
isolation and L3 tunneling for interconnecting instances of the same
VNI on NVEs.
In the case of L3 VNI, the inner TTL field MUST be decremented by
(at least) 1 as if the NVO3 egress NVE was one (or more) hop(s)
away. The TTL field in the outer IP header MUST be set to a value
appropriate for delivery of the encapsulated frame to the tunnel
exit point. Thus, the default behavior MUST be the TTL pipe model
where the overlay network looks like one hop to the sending NVE.
Configuration of a "uniform" TTL model where the outer tunnel TTL is
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set equal to the inner TTL on ingress NVE and the inner TTL is set
to the outer TTL value on egress MAY be supported. [RFC2983]
provides additional details on the uniform and pipe models.
L2 and L3 VNIs can be deployed in isolation or in combination to
optimize traffic flows per tenant across the overlay network. For
example, an L2 VNI may be configured across a number of NVEs to
offer L2 multi-point service connectivity while a L3 VNI can be co-
located to offer local routing capabilities and gateway
functionality. In addition, integrated routing and bridging per
tenant MAY be supported on an NVE. An instantiation of such service
may be realized by interconnecting an L2 VNI as access to an L3 VNI
on the NVE.
When underlay multicast is supported, it MAY be possible to select
whether the NVE provides optimized underlay multicast trees inside
the VNI for individual tenant multicast groups or whether a default
underlay VNI multicasting tree, where all the NVEs of the
corresponding VNI are members, is used.
3.3. Overlay Module
The overlay module performs a number of functions related to NVO3
header and tunnel processing.
The following figure shows a generic NVO3 encapsulated frame:
+--------------------------+
| Tenant Frame |
+--------------------------+
| NVO3 Overlay Header |
+--------------------------+
| Outer Underlay header |
+--------------------------+
| Outer Link layer header |
+--------------------------+
Figure 2 : NVO3 encapsulated frame
where
. Tenant frame: Ethernet or IP based upon the VNI type
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. NVO3 overlay header: Header containing VNI context information
and other optional fields that can be used for processing
this packet.
. Outer underlay header: Can be either IP or MPLS
. Outer link layer header: Header specific to the physical
transmission link used
3.3.1. NVO3 overlay header
An NVO3 overlay header MUST be included after the underlay tunnel
header when forwarding tenant traffic.
Note that this information can be carried within existing protocol
headers (when overloading of specific fields is possible) or within
a separate header.
3.3.1.1. Virtual Network Context Identification
The overlay encapsulation header MUST contain a field which allows
the encapsulated frame to be delivered to the appropriate virtual
network endpoint by the egress NVE.
The egress NVE uses this field to determine the appropriate virtual
network context in which to process the packet. This field MAY be an
explicit, unique (to the administrative domain) virtual network
identifier (VNID) or MAY express the necessary context information
in other ways (e.g. a locally significant identifier).
In the case of a global identifier, this field MUST be large enough
to scale to 100's of thousands of virtual networks. Note that there
is typically no such constraint when using a local identifier.
3.3.1.2. Quality of Service (QoS) identifier
Traffic flows originating from different applications could rely on
differentiated forwarding treatment to meet end-to-end availability
and performance objectives. Such applications may span across one or
more overlay networks. To enable such treatment, support for
multiple Classes of Service (Cos) across or between overlay networks
MAY be required.
To effectively enforce CoS across or between overlay networks
without Deep Packet Inspection (DPI) repeat, NVEs MAY be able to map
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CoS markings between networking layers, e.g., Tenant Systems,
Overlays, and/or Underlay, enabling each networking layer to
independently enforce its own CoS policies. For example:
- TS (e.g. VM) CoS
o Tenant CoS policies MAY be defined by Tenant administrators
o QoS fields (e.g. IP DSCP and/or Ethernet 802.1p) in the
tenant frame are used to indicate application level CoS
requirements
- NVE CoS: Support for NVE Service CoS MAY be provided through a
QoS field, inside the NVO3 overlay header
o NVE MAY classify packets based on Tenant CoS markings or
other mechanisms (eg. DPI) to identify the proper service CoS
to be applied across the overlay network
o NVE service CoS levels are normalized to a common set (for
example 8 levels) across multiple tenants; NVE uses per
tenant policies to map Tenant CoS to the normalized service
CoS fields in the NVO3 header
- Underlay CoS
o The underlay/core network MAY use a different CoS set (for
example 4 levels) than the NVE CoS as the core devices MAY
have different QoS capabilities compared with NVEs.
o The Underlay CoS MAY also change as the NVO3 tunnels pass
between different domains.
3.3.2. Tunneling function
This section describes the underlay tunneling requirements. From an
encapsulation perspective, IPv4 or IPv6 MUST be supported, both IPv4
and IPv6 SHOULD be supported, MPLS MAY be supported.
3.3.2.1. LAG and ECMP
For performance reasons, multipath over LAG and ECMP paths MAY be
supported.
LAG (Link Aggregation Group) [IEEE 802.1AX-2008] and ECMP (Equal
Cost Multi Path) are commonly used techniques to perform load-
balancing of microflows over a set of a parallel links either at
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Layer-2 (LAG) or Layer-3 (ECMP). Existing deployed hardware
implementations of LAG and ECMP uses a hash of various fields in the
encapsulation (outermost) header(s) (e.g. source and destination MAC
addresses for non-IP traffic, source and destination IP addresses,
L4 protocol, L4 source and destination port numbers, etc).
Furthermore, hardware deployed for the underlay network(s) will be
most often unaware of the carried, innermost L2 frames or L3 packets
transmitted by the TS.
Thus, in order to perform fine-grained load-balancing over LAG and
ECMP paths in the underlying network, the encapsulation needs to
present sufficient entropy to exercise all paths through several
LAG/ECMP hops.
The entropy information can be inferred from the NVO3 overlay header
or underlay header. If the overlay protocol does not support the
necessary entropy information or the switches/routers in the
underlay do not support parsing of the additional entropy
information in the overlay header, underlay switches and routers
should be programmable, i.e. select the appropriate fields in the
underlay header for hash calculation based on the type of overlay
header.
All packets that belong to a specific flow MUST follow the same path
in order to prevent packet re-ordering. This is typically achieved
by ensuring that the fields used for hashing are identical for a
given flow.
The goal is for all paths available to the overlay network to be
used efficiently. Different flows should be distributed as evenly as
possible across multiple underlay network paths. For instance, this
can be achieved by ensuring that some fields used for hashing are
randomly generated.
3.3.2.2. DiffServ and ECN marking
When traffic is encapsulated in a tunnel header, there are numerous
options as to how the Diffserv Code-Point (DSCP) and Explicit
Congestion Notification (ECN) markings are set in the outer header
and propagated to the inner header on decapsulation.
[RFC2983] defines two modes for mapping the DSCP markings from inner
to outer headers and vice versa. The Uniform model copies the inner
DSCP marking to the outer header on tunnel ingress, and copies that
outer header value back to the inner header at tunnel egress. The
Pipe model sets the DSCP value to some value based on local policy
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at ingress and does not modify the inner header on egress. Both
models SHOULD be supported.
[RFC6040] defines ECN marking and processing for IP tunnels.
3.3.2.3. Handling of BUM traffic
NVO3 data plane support for either ingress replication or point-to-
multipoint tunnels is required to send traffic destined to multiple
locations on a per-VNI basis (e.g. L2/L3 multicast traffic, L2
broadcast and unknown unicast traffic). It is possible that both
methods be used simultaneously.
There is a bandwidth vs state trade-off between the two approaches.
User-configurable settings MUST be provided to select which
method(s) gets used based upon the amount of replication required
(i.e. the number of hosts per group), the amount of multicast state
to maintain, the duration of multicast flows and the scalability of
multicast protocols.
When ingress replication is used, NVEs MUST maintain for each VNI
the related tunnel endpoints to which it needs to replicate the
frame.
For point-to-multipoint tunnels, the bandwidth efficiency is
increased at the cost of more state in the Core nodes. The ability
to auto-discover or pre-provision the mapping between VNI multicast
trees to related tunnel endpoints at the NVE and/or throughout the
core SHOULD be supported.
3.4. External NVO3 connectivity
It is important that NVO3 services interoperate with current VPN and
Internet services. This may happen inside one DC during a migration
phase or as NVO3 services are delivered to the outside world via
Internet or VPN gateways (GW).
Moreover the compute and storage services delivered by a NVO3 domain
may span multiple DCs requiring Inter-DC connectivity. From a DC
perspective a set of GW devices are required in all of these cases
albeit with different functionalities influenced by the overlay type
across the WAN, the service type and the DC network technologies
used at each DC site.
A GW handling the connectivity between NVO3 and external domains
represents a single point of failure that may affect multiple tenant
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services. Redundancy between NVO3 and external domains MUST be
supported.
3.4.1. Gateway (GW) Types3.4.1.1. VPN and Internet GWs
Tenant sites may be already interconnected using one of the existing
VPN services and technologies (VPLS or IP VPN). If a new NVO3
encapsulation is used, a VPN GW is required to forward traffic
between NVO3 and VPN domains. Internet connected Tenants require
translation from NVO3 encapsulation to IP in the NVO3 gateway. The
translation function SHOULD minimize provisioning touches.
3.4.1.2. Inter-DC GW
Inter-DC connectivity MAY be required to provide support for
features like disaster prevention or compute load re-distribution.
This MAY be provided via a set of gateways interconnected through a
WAN. This type of connectivity MAY be provided either through
extension of the NVO3 tunneling domain or via VPN GWs.
3.4.1.3. Intra-DC gateways
Even within one DC there may be End Devices that do not support NVO3
encapsulation, for example bare metal servers, hardware appliances
and storage. A gateway device, e.g. a ToR switch, is required to
translate the NVO3 to Ethernet VLAN encapsulation.
3.4.2. Path optimality between NVEs and Gateways
Within an NVO3 overlay, a default assumption is that NVO3 traffic
will be equally load-balanced across the underlying network
consisting of LAG and/or ECMP paths. This assumption is valid only
as long as: a) all traffic is load-balanced equally among each of
the component-links and paths; and, b) each of the component-
links/paths is of identical capacity. During the course of normal
operation of the underlying network, it is possible that one, or
more, of the component-links/paths of a LAG may be taken out-of-
service in order to be repaired, e.g.: due to hardware failure of
cabling, optics, etc. In such cases, the administrator may configure
the underlying network such that an entire LAG bundle in the
underlying network will be reported as operationally down if there
is a failure of any single component-link member of the LAG bundle,
(e.g.: N = M configuration of the LAG bundle), and, thus, they know
that traffic will be carried sufficiently by alternate, available
(potentially ECMP) paths in the underlying network. This is a likely
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an adequate assumption for Intra-DC traffic where presumably the
costs for additional, protection capacity along alternate paths is
not cost-prohibitive. In this case, there are no additional
requirements on NVO3 solutions to accommodate this type of
underlying network configuration and administration.
There is a similar case with ECMP, used Intra-DC, where failure of a
single component-path of an ECMP group would result in traffic
shifting onto the surviving members of the ECMP group.
Unfortunately, there are no automatic recovery methods in IP routing
protocols to detect a simultaneous failure of more than one
component-path in a ECMP group, operationally disable the entire
ECMP group and allow traffic to shift onto alternative paths. This
problem is attributable to the underlying network and, thus, out-of-
scope of any NVO3 solutions.
On the other hand, for Inter-DC and DC to External Network cases
that use a WAN, the costs of the underlying network and/or service
(e.g.: IPVPN service) are more expensive; therefore, there is a
requirement on administrators to both: a) ensure high availability
(active-backup failover or active-active load-balancing); and, b)
maintaining substantial utilization of the WAN transport capacity at
nearly all times, particularly in the case of active-active load-
balancing. With respect to the dataplane requirements of NVO3
solutions, in the case of active-backup fail-over, all of the
ingress NVE's need to dynamically adapt to the failure of an active
NVE GW when the backup NVE GW announces itself into the NVO3 overlay
immediately following a failure of the previously active NVE GW and
update their forwarding tables accordingly, (e.g.: perhaps through
dataplane learning and/or translation of a gratuitous ARP, IPv6
Router Advertisement). Note that active-backup fail-over could be
used to accomplish a crude form of load-balancing by, for example,
manually configuring each tenant to use a different NVE GW, in a
round-robin fashion.
3.4.2.1. Load-balancing
When using active-active load-balancing across physically separate
NVE GW's (e.g.: two, separate chassis) an NVO3 solution SHOULD
support forwarding tables that can simultaneously map a single
egress NVE to more than one NVO3 tunnels. The granularity of such
mappings, in both active-backup and active-active, MUST be specific
to each tenant.
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L2/ELAN over NVO3 service may span multiple racks distributed across
different DC regions. Multiple ELANs belonging to one tenant may be
interconnected or connected to the outside world through multiple
Router/VRF gateways distributed throughout the DC regions. In this
scenario, without aid from an NVO3 or other type of solution,
traffic from an ingress NVE destined to External gateways will take
a non-optimal path that will result in higher latency and costs,
(since it is using more expensive resources of a WAN). In the case
of traffic from an IP/MPLS network destined toward the entrance to
an NVO3 overlay, well-known IP routing techniques MAY be used to
optimize traffic into the NVO3 overlay, (at the expense of
additional routes in the IP/MPLS network). In summary, these issues
are well known as triangular routing (a.k.a. traffic tromboning).
Procedures for gateway selection to avoid triangular routing issues
SHOULD be provided.
The details of such procedures are, most likely, part of the NVO3
Management and/or Control Plane requirements and, thus, out of scope
of this document. However, a key requirement on the dataplane of any
NVO3 solution to avoid triangular routing is stated above, in
Section 3.4.2, with respect to active-active load-balancing. More
specifically, an NVO3 solution SHOULD support forwarding tables that
can simultaneously map a single egress NVE to more than one NVO3
tunnel.
The expectation is that, through the Control and/or Management
Planes, this mapping information may be dynamically manipulated to,
for example, provide the closest geographic and/or topological exit
point (egress NVE) for each ingress NVE.
3.5. Path MTU
The tunnel overlay header can cause the MTU of the path to the
egress tunnel endpoint to be exceeded.
IP fragmentation SHOULD be avoided for performance reasons.
The interface MTU as seen by a Tenant System SHOULD be adjusted such
that no fragmentation is needed. This can be achieved by
configuration or be discovered dynamically.
Either of the following options MUST be supported:
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o Classical ICMP-based MTU Path Discovery [RFC1191] [RFC1981] or
Extended MTU Path Discovery techniques such as defined in
[RFC4821]
o Segmentation and reassembly support from the overlay layer
operations without relying on the Tenant Systems to know about
the end-to-end MTU
o The underlay network MAY be designed in such a way that the MTU
can accommodate the extra tunnel overhead.
3.6. Hierarchical NVE dataplane requirements
It might be desirable to support the concept of hierarchical NVEs,
such as spoke NVEs and hub NVEs, in order to address possible NVE
performance limitations and service connectivity optimizations.
For instance, spoke NVE functionality may be used when processing
capabilities are limited. In this case, a hub NVE MUST provide
additional data processing capabilities such as packet replication.
3.7. Other considerations3.7.1. Data Plane Optimizations
Data plane forwarding and encapsulation choices SHOULD consider the
limitation of possible NVE implementations, specifically in software
based implementations (e.g. servers running virtual switches)
NVE SHOULD provide efficient processing of traffic. For instance,
packet alignment, the use of offsets to minimize header parsing,
padding techniques SHOULD be considered when designing NVO3
encapsulation types.
The NV03 encapsulation/decapsulation processing in software-based
NVEs SHOULD make use of hardware assist provided by NICs in order to
speed up packet processing.
3.7.2. NVE location trade-offs
In the case of DC traffic, traffic originated from a VM is native
Ethernet traffic. This traffic can be switched by a local VM switch
or ToR switch and then by a DC gateway. The NVE function can be
embedded within any of these elements.
The NVE function can be supported in various DC network elements
such as a VM, VM switch, ToR switch or DC GW.
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